A Current Optimization Strategy for a Hybrid DC Transformer with MMC Structure and Series-connected Switches

A DC transformer utilizing modular multilevel converter (MMC) structure and series-connected switches incurs high current stress and results in low system efficiency under low-voltage and light-load conditions. An optimization control strategy utilizing the phase shift on the low-voltage (LV)-side full bridge is proposed. It is shown that by introducing the zero state to the output voltage of the LV-side full bridge the spikes in inductor current are eliminated thus reducing the current stress of switches and system power losses. While maintaining the power regulation capability of the circuit optimal current stress is achieved under different operating modes by segmented phase shift angel control. The soft-switching characteristic of power switches is unchanged after applying this strategy. The proposed control strategy is verified by simulation and experimental experiment results.